华北克拉通栖霞地区~2.9Ga变质基性火山岩锆石SHRIMP U-Pb定年、Hf-O同位素特征及构造意义

Characteristic and tectonic significance of zircon SHRIMP U-Pb geochronology and Hf-O isotopes from~2.9Ga metamorphic basic volcanics in Qixia area,North China Craton

胶东地区是华北克拉通重要的早前寒武纪研究区之一。中太古代唐家庄岩群在形成时代、岩石成因及构造环境等方面一直少有研究。作者在栖霞地区对出露的唐家庄岩群变质表壳岩进行系统地质调查研究基础上,开展了变质基性火山岩全岩地球化学、锆石SHRIMP U-Pb和Hf-O同位素等分析研究。结果显示,变质基性火山岩岩浆锆石年龄为2907.9±6.3Ma,代表了中太古代唐家庄岩群的形成时间,并遭受新太古代早、晚期变质作用。这是~2.9Ga唐家庄岩群变质基性火山岩在胶东地区首次报道。岩石地球化学特征表现为相对富集大离子亲石元素K、Rb、Sr等,Th、Nb、Zr、Hf等高场强元素相对亏损,具有中等分异的REE配分模式。岩石中岩浆锆石具有相似的Hf、O同位素组成,εHf(t)值介于-0.68~+4.21,δ18 O值介于1.38‰~6.73‰。结合前人的工作,我们得出的主要结论如下:(1)胶东地区经历~2.9Ga岩浆事件,局部地段仍保留下了同时期的变质表壳岩;(2)唐家庄岩群是中太古代初始的洋盆环境中所形成的一套火山-沉积建造;(3)华北克拉通中太古代晚期是亏损地幔新生地壳加积的主要时期,局部地壳再循环了较老的陆源物质;(4)华北克拉通新太古代晚期经历了伸展构造背景,岩浆底侵作用使岩石发生强烈变质作用。

Jiaodong area is one of the important Early Precambrian research areas in the North China Craton.There have been few studies on the formation age,petrogenesis and tectonic environment of the Tangjiazhuang Group in the Middle Archaean.Based on the systematic geological investigation of metamorphic basic volcanics of the Tangjiazhuang Group in Qixia area,the authors have carried out whole-rock geochemistry analysis of plagioclase amphibolite,zircon SHRIMP U-Pb chronology and Hf-O isotopes.The results show that the zircon age of the metamorphic basic volcanics in the Tangjiazhuang Group is 2907.9±6.3Ma,which represents that rock assemblages of the Tangjiazhuang Group initially formed in the Mid-Archaean and underwent the early and late Neo-Archaean metamorphism.This is the first report of~2.9Ga metamorphic basic volcanic rocks of Tangjiazhuang Group in Jiaodong area.The geochemical characteristics of the rocks are relatively enriched in large ion lithophile elements K,Rb,Sr,etc.,and depleted in high field strength elements such as Th,Nb,Zr,Hf,etc.,and REE pattern displays medium-degree depletion.The magmatic zircons have similar Hf and O isotopic compositions withεHf(t)values ranging from-0.68 to+4.21 andδ18 O values ranging from 1.38‰to 6.73‰.Based on the previous work,this paper proposes main conclusions as follows:(1)Jiaodong area experienced~2.9Ga magmatic event,and some parts still retain metamorphosed supracrustal rocks of the same period;(2)The Tangjiazhuang Group is a set of volcano-sedimentary formations formed in the initial ocean basin environment of the Mid-Archaean;(3)Accretionary neo-crust derived from depleted mantle of the North China Craton occurred mainly from the late Middle Archaean Period,and partially recycled terrigenous materials;(4)The North China Craton experienced extensional tectonic in the late Neo-Archaean Period,and magma floor invasion caused strong metamorphism of rocks.